Explosive composition and method for production thereof

ABSTRACT

An explosive composition is adapted for use in a safety arrangement for vehicles, and comprises a fuel of a micro-structured or nano-structured porous solid material and an oxidizing agent which is solid or liquid at ambient temperature. The oxidizing agent is selected from the group consisting of sulphur, selenium, tellurium, bromine, iodine, phosphorus and arsenic as well as mixtures and oxygen-free compounds thereof. In a method for the production of the explosive composition, the oxidizing agent is dissolved in a solvent and introduced into the pores of the fuel.

TECHNICAL FIELD

The invention relates to an explosive composition for use in a safety arrangement for vehicles, with a fuel of a micro- or nano-structured porous solid material and with an oxidizing agent which is solid or liquid at ambient temperature.

BACKGROUND OF THE INVENTION

A generic explosive composition is known from DE 102 04 834 A1. In the prior art composition, the oxidizing agent which is solid or liquid at ambient temperature is introduced into the pores of the porous fuel. At least 50% of the oxidizing agent is selected from the group of organic nitro-compounds or nitrates, alkali metal nitrates or alkaline earth metal nitrates, metal nitrates, metal chlorates, metal perchlorates, metal bromates, metal iodates, metal oxides, metal peroxides, ammonium perchlorate, ammonium nitrate, hydrogen peroxide and hydroxyl ammonium nitrate. The known composition is suitable in particular for use as igniting composition.

Furthermore, DE 102 04 895 discloses a nano-structured, porous, reactive substance which consists of reactive bodies and has cavities which are sized in a range of magnitude of 1 to 1000 nm and are provided with oxidizing agents, the reactive substance consisting of reactive particles which are independent of each other and surrounded by a protective layer. The reactive bodies may consist of silicon, boron, aluminium, titanium or zirconium. The oxidizing agents are in particular alkali metal nitrates and alkaline earth metal nitrates and also further oxygen-containing oxidizing agents.

Finally DE 101 62 413 A1 discloses an integrated explosive element or ignition element, which has a base body of silicon and a reaction region associated therewith, the reaction region comprising porous silicon and an oxidizing agent for silicon. As the oxidizing agent, inorganic or organic compounds are proposed, which release oxygen, fluorine, chlorine or other oxidizing substances on heating. In particular, inorganic nitrates and inorganic peroxides and also further oxygen-containing salts are mentioned as examples. The chemical reaction between the oxidizing agent and the porous silicon is initiated by heating by means of current-carrying conductors. The integrated explosive element or ignition element is intended for use in a microreactor, a microbooster for course correction of satellites, as an ignition element in gas generators for a belt tensioner or an airbag, or as an initial ignition element for the ignition of explosive charges.

Several of the oxidizers proposed in the prior art for use with porous silicon are, however, hydroscopic and/or form modifications containing water of crystallization. Thereby, however, the storage stability of the compositions can be affected disadvantageously. Also, these oxidizers only display a low solubility in organic solvents or have a high melting point. The filling of the porous fuel must therefore take place in several stages or under increased safety provisions. Owing to the high viscosity of the salt melts, the filling of the pores with the oxidizing agent is also often incomplete in this case. Therefore, however, difficulties result with the precise setting of the ratio between porous fuel and oxidizing agents. Therefore, the explosive characteristics of the resulting compositions can vary over a wide range and can be standardized only with difficulty. A range of oxidizing agents indicated in the prior art can, in addition, not be obtained purely. The impurities contained in these oxidizing agents likewise impair the explosive behaviour of the compositions produced therewith.

SUMMARY OF THE INVENTION

An object of the invention is to avoid the above-mentioned disadvantages and to provide a stable, explosive composition which can be produced at a favourable cost and in particular can be used for civil applications.

According to the invention, an explosive composition is provided for use in a safety arrangement for vehicles, the composition comprising a fuel of a micro-structured or nano-structured porous solid material and an oxidizing agent which is solid or liquid at ambient temperature, wherein the oxidizing agent is selected from the group consisting of sulphur, selenium, tellurium, bromine, iodine, phosphorus and arsenic, and also mixtures and oxygen-free compounds thereof. Preferably, the composition according to the invention consists of the fuel and the oxidizing agent.

The oxidizing agents to be used according to the invention display a high bonding energy to silicon and, in so doing, also a sufficiently high heat of explosion. They are, in addition, easily able to be vaporized or sublimable and can therefore be readily used in methods for chemical or physical vapor deposition (CVD or PVD methods). A range of oxidizing agents to be used according to the invention is, furthermore, readily soluble in non-polar, volatile organic solvents. These oxidizing agents, such as sulphur and iodine for example dissolve, furthermore, substantially better in the likewise non-polar solvent carbon dioxide than the polar oxygen salts. Therefore, the oxidizing agents to be used according to the invention can be introduced very simply into the pores of the micro-structured or nano-structured fuel using supercritical carbon dioxide. After the vaporizing off of the solvent, only the oxidizing agent remains, free of residue, in the porous structure of the fuel.

With the use of sulphur as oxidizing agent, owing to the low melting point of 113° C., the direct introduction of the oxidizing agent in the melted state into the pores of the micro-structured or nano-structured fuel is possible without impurities.

The above oxidizing agents can therefore be introduced substantially more easily in stoichiometric quantities into the nano-structured fuel. They provide at the same time a high heat of explosion and are able to be handled well in the filling of the pores of the nano-structured fuel.

Conventional oxygen-containing and salt-like oxidizing agents are distinguished in addition by a more or less marked hygroscopicity. These substances therefore require a high degree of effort in terms of process technology, because the presence of water or humidity in the air must be reliably ruled out. Furthermore, the compositions produced with these substances must be hermetically sealed, in order to ensure their capability of functioning over the entire lifespan of the structure of up to 15 years. Through the use of the oxidizing agents according to the invention, in particular of the non-hygroscopic sulphur, these disadvantages are also reliably eliminated.

A further object of the invention is a method for the production of the explosive composition according to the invention, which is characterized in that the oxidizing agent is dissolved in a solvent and introduced into the pores of the nanostructured fuel. In particular, the use of non-polar solvent ensures a good solubility of the likewise non-polar oxidizing agent according to the present invention. The solvent is in addition to be able to be vaporized, free of residue, from the porous fuel structure. Thereby, the adjusting of stoichiometric compositions of the fuel and the oxidizing agent is substantially facilitated. In particular, supercritical carbon dioxide, carbon disulfide, tetrachloromethane and also aromatic and saturated aliphatic hydrocarbons are suitable as solvents. Generally it can be assumed that solvents with a Reichardt polarity of E_(T)(30)/kcal/mol≧50 can be used.

The micro-structured or nano-structured fuel is preferably a solid body with a sponge-like structure of amorphous, partially crystalline or crystalline particles with a structure size of between approximately 2 nm and 1000 nm, and has a porosity (V_(pores)/V_(sample)) of between 10% and 98%, preferably between 40 and 80%. The fuel can have a specific surface of up to 1000 m²/cm³, preferably between 200 and 1000 m²/cm³.

The structure size and the size and form of the pores can be varied in a wide range. The structure size indicates the average size of the particles constituting the fuel, and preferably lies in a range of about 2 to 50 nm, particularly preferably between about 2 nm and 10 nm. The pore size preferably ranges between 2 nm and 1000 nm.

The porous fuel is preferably a semiconductor material, and particularly preferably selected from the group consisting of Si, Ge, SiGe, SiC, InP and GaAs. The production of micro-structured or nano-structured porous materials from these substances is described in the scientific literature. Suitable in particular as production methods are chemical or physical deposition methods, such as electrochemical deposition, CVD, PVD or sputtering or the pressing of nano-fine particles. In the case of silicon, these nano-fine particles are obtainable by slow burning of silane.

Particularly preferably, the fuel is so-called “porous silicon” which can be produced particularly simply by electrochemical etching of silicon in fluoride-containing solutions. The use of porous semiconductor materials, e.g. silicon, makes possible the simple integration into known semiconductor components using conventional semiconductor process techniques.

Advantageously, the porous fuel is at least partially passivated, which means the inner surface of the fuel is at least partially saturated with oxygen or altered in another way so that an activation energy to be overcome for reaction with the oxidizer is increased. The passivation can take place for example by heating the fuel in an oxygen-containing atmosphere or air. Through the passivation, a further adjustability of the pyrotechnic characteristics of the composition according to the invention is possible, such as for example its ignitability by electrical discharge or the action of UV light.

As the chemical reaction of the porous fuel takes place from the surface, the activation energy to be overcome for the igniting of the fuel can be increased by means of a less reactive protective layer on the surface of the nanoparticles. This passivation layer can be subsequently applied onto the porous fuel and can consist of an inert material (e.g. Teflon). The passivation layer can also be formed by means of thermal, chemical, physical or electrochemical treatment of the fuel.

A stable passivation layer can be formed for example by tempering the porous silicon in air, preferably following the electrochemical etching step and before the filling of the pores with the oxidizer. If the tempering takes place in the range from between 150 degrees C. and 300 degrees C., preferably at approximately 200 degrees C., after up to approximately 1600 minutes an oxygen submono-layer of silicon/oxygen bonds (Si—O) forms, which have a higher bonding energy than the silicon/hydrogen bonds. The surface of the silicon nano-crystals then consists, after tempering, of H—Si—O-complexes, because at approximately 200 degrees C. the hydrogen is retained on the surface of the nano particles and oxygen is bonded to silicon under the first mono-layer. If the tempering is carried out at temperatures above approximately 300 degrees C. (e.g. 700 degrees C. for, 30 seconds), the hydrogen is driven off from the surface of the nano-particles and layers of “pure” Si—O bonds form. Samples tempered in such a way and filled with oxidizing agent are extremely stable and secure in handling, but may nevertheless be brought to explosion by means of a sudden heating.

The passivation of the surface of the porous fuel also increases the long term stability of the explosive composition, because a time-dependent change to the surface characteristics of the fuel under the influence of the oxidizing agent can no longer occur.

The oxidizing agent preferably consists in whole or in part of iodine, sulphur or oxygen-free sulphur compounds. These oxidizing agents are easily soluble in non-polar organic solvents and can be introduced free of reside into the porous fuel structure. They are, in addition, also stable in storage when combined with non-passivated porous silicon. With these oxidizing agents, therefore, according to the existing requirements, the production of the passivation layer described above can be dispensed with.

The oxidizer and the fuel can be present for instance in a stoichiometric ratio. According to the purpose of application, the oxidizer can, however, also be over-balanced or under-balanced in relation to the fuel.

The composition according to the invention has, in addition, a high structural strength, because the fuel is present as a solid, dimensionally stable matrix. The composition can therefore be used as a supporting component in pyrotechnic objects, e.g. igniters. In addition, the production processes known from semiconductor technology and micromechanics are able to be used. Thereby, the possibility exists for favourably priced production using standard components. In particular, the complete integration of the composition according to the invention in semiconductor circuits is made possible.

The use of the composition according to the invention as component of an igniter is therefore also an object of the invention. This igniter can be advantageously integrated in a semiconductor circuit. In particular, the igniter can be a component of a safety system in vehicles, such as for example of a gas generator for a belt tensioner or a gas bag module.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Further features and advantages of the invention will be apparent from the following description of preferred embodiments.

For the production of the explosive composition according to the invention, firstly porous, nanostructured silicon is provided by electrochemical etching in accordance with the method described in Materials Science and Engineering B 69-70 (2000) 11-22 or Phys. Rev. Lett. (2001), 87, 68 301 ff. A silicon substrate was connected as anode in an etching cell and treated in an electrolyte containing hydrofluoric acid, for example a mixture of equal volume proportions of ethanol and concentrated hydrofluoric acid (50%) with an anodizing current of between 20 and 70 mA/cm². The porosity of the resulting porous silicon was in the range between 40% and 80%. The structure size varied between 2 and 10 nm.

The porous silicon thus obtained was tempered for 26 h at 200 degrees C. in air, then impregnated with a saturated solution of sulphur in carbon disulphide and subsequently dried in air. By means of an electrical spark, a powerful explosion was able to be triggered. In a storage test at 104 degrees C. for 400 hours, the composition did not show any substantial increase in weight.

In further tests, tempered and untempered samples of the above porous silicon were filled with sulphur from the melting state. The sulphur was put in the solid state on the samples of porous silicon, and the samples were heated on a heating plate to about 125±5° C. to form melted sulphur. The melted sulphur was kept on the sample for about 30 seconds allowing the melted sulphur to enter the pores of the porous silicon. Thereafter, the excess sulphur was removed from the sample body. In this manner, filling degrees of more than 90% could be obtained, as calculated from gravimetric measurements.

For reducing the melting point and/or the viscosity of the sulphur melt, further additives, such as ethylene glycol or sugar, can be admixed to the sulphur. The samples can be filled with the sulphur from the melting state in air or in vacuum. It is further possible to fill the samples of tempered or untempered porous silicon with sulfur through sublimation in a vacuum chamber or by means of physical vapor deposition of sulphur.

All of the samples of porous silicon which were filled with sulphur according to the above methods could be brought to explosion by rapid heating on a heating plate or by means of an electrically heated ignition bridge.

Use of melted sulphur for filling samples of tempered porous silicon resulted in samples having an ignition temperature ranging between 239 and 267° C. Comparative values for samples of tempered porous silicon filled with calcium perchlorate or sodium perchlorate obtained by digital scanning calorimetric measurements (DSC) were in the range of between 185 to 210° C. and 208 to 237° C., respectively.

The results show that the system of porous silicon/sulphur is suitable for use as an explosive material. The intensity of the explosion can be controlled by means of the porosity of the porous silicon, because the pore volume controls the quantity of the introduced oxidizing agent and hence the stoichiometry of the reaction partners. The oxidation, however, does not take place spontaneously, but rather can be specifically triggered, for example, by a current impulse. In addition, the samples consisting of porous silicon filled with sulphur have a high mechanical and chemical stability and are thus extremely safe in handling. For example, the samples will maintain their complete workability even if they are divided using a conventional water cooled wafer saw in wafer processing.

The composition according to the invention can be used in particular in an igniter of safety arrangements for vehicles, for example gas bag modules or belt tensioners. Such igniters can be produced advantageously by known methods of semiconductor or silicon process technology. In particular, a simple and favourably priced production with high precision is already possible in batch process on the wafer level. 

1. An explosive composition for use in a safety arrangement for vehicles, the composition comprising a fuel of a micro-structured or nano-structured porous solid material and an oxidizing agent which is solid or liquid at ambient temperature, wherein the oxidizing agent is selected from the group consisting of sulphur, selenium, tellurium, bromine, iodine, phosphorus and arsenic as well as mixtures and oxygen-free compounds thereof.
 2. The explosive composition according to claim 1, wherein the oxidizing agent is selected from the group consisting of iodine, sulphur or oxygen-free sulphur compounds.
 3. The explosive composition according to claim 1, wherein the fuel is porous silicon.
 4. A method for producing an explosive composition wherein the composition comprises a fuel of a micro-structured or nano-structured porous solid material and an oxidizing agent which is solid or liquid at ambient temperature, and wherein the oxidizing agent is selected from the group consisting of sulphur, selenium, tellurium, bromine, iodine, phosphorus and arsenic as well as mixtures and oxygen-free compounds thereof, the method comprising the step of dissolving the oxidizing agent in a solvent and introducing the dissolved oxidizing agent into the pores of the fuel.
 5. The method according to claim 4, wherein the solvent is supercritical carbon dioxide.
 6. The method according to claim 4, wherein the solvent is carbon disulphide.
 7. A method for producing an explosive composition, wherein the composition comprises a fuel of a micro-structured or nanostructured porous solid material and an oxidizing agent, wherein the oxidizing agent is sulphur, and wherein the method comprises the steps of: providing the porous fuel; and introducing the sulphur from the melting state into the pores of the porous fuel.
 8. The method according to claim 7, wherein the sulphur is heated to about 125±5° C. to form the melted state.
 9. The method according to claim 4, wherein the step of providing the porous fuel comprises the step of anodically etching of silicon in a fluoride containing solution to form porous silicon.
 10. The method according to claim 4, further comprising the step of passivating the porous fuel before the introduction of the oxidizer into the pores.
 11. A method of activating a safety arrangement in vehicles, wherein the safety arrangement includes an igniter component, and wherein the igniter component comprises the explosive composition of claim
 1. 